1. Field of the Invention
This invention concerns a multi-band electronic tuning radio receiver capable of selecting channels in various systems such as digital channel selection, search channel selection and manual channel selection.
2. Background of Art
Electronic tuning, that is, electronic channel selection can be executed using electrically controllable variable reactance elements such as voltage-variable capacitance diodes in tuning circuits of a radio receiver and by providing a circuit for generating a bias voltage for controlling the above elements and, for the channel selection method, various systems are usable such as a search system adapted to sweep a broadcasting frequency band by the generation of a gradually increasing or decreasing control voltage, a digital system adapted to set a frequency for a broadcasting station in a digital manner and change a control voltage as far as the above frequency is reached or a manual system adapted to execute the channel selection by changing a control voltage manually. The applicant has already proposed for these systems such as in Japanese Patent Application Nos. 39648/1976, 40024/1976, 129831/1976 and 70270/1977.
The radio receiver of this type has a basic structure, as shown in FIG. 1, in which in a usual superheterodyne type radio receiver consisting of an antenna 10, a high frequency amplifying stage 12, a local oscillation stage 14, an intermediate frequency amplifying stage 16, a detection stage 18, a low frequency amplifying stage 20 and a speaker 22, voltage-variable reactance elements 11 and 13 are used in the tuning circuits for the high frequency amplifying stage and the local oscillation stage, and moreover a voltage memory 24 for supplying a control voltage (bias voltage) to the elements, a control circuit 26 for the memory, a counter 28, a comparator 30, channel selection switches 32, a frequency code memory 34 and a frequency display section 36 are provided to the receiver. The voltage-variable reactance elements are, for example, made of voltage-variable capacitance diodes. The voltage memory basically consists of an integrating circuit provided with over a long time period.
The outline of the operation is as follows. Upon push of a manual channel selection switch in the selection switches 32, the control circuit 26 outputs an upward (positive) or a downward (negative) signal Iu or Id, by which the voltage memory 24 generates a gradually increasing or decreasing bias voltage to change the capacitance in the variable reactance elements 11 and 13. When a broadcasting wave is received and the broadcasting issues from the speaker 22, an audience hears it and, if it is a desired one, stops the push for the switch to enter to a receiving state. The audience keeps to push the switch, if it is not a desired one, and searches the next broadcasting station. Upon search channel selection, the search channel selection switch in the selection switches is pushed temporarily. This causes the flip-flops in the control circuit 26 to be set to output the signal Iu or Id. When a broadcasting wave is received, the flip-flops are reset by the intermediate frequency output from the intermediate frequency amplifying stage 16 and the signal Iu or Id is eliminated. If the broadcasting wave is a desired one the receiver enters into the receiving state as it is and, if not, the switch may be pushed again. Also upon digital channel selection, a predetermined digital channel selection switch in the selection switches 32 is pushed. This reads out the code for a predetermined broadcasting station from the memory 34 and it is compared in the comparator 30 with a value counted on the counter 28 for counting the local oscillation frequency of the local oscillator 14. The control circuit 26 outputs the upward or the downward signal Iu or Id till both of them coincide and, when the coincidence is attained, the signals are eliminated to proceed to the receiving state.
While the above radio receiver is constructed as a multi-band type receiver capable of listening to FM broadcasting as well as AM broadcasting, the following problems are resulted upon band switching. Channel selection is executed by the increase or decrease in the control voltage outputted from the voltage memory 24 to thereby vary the capacitance of the element 11 and 13, and a curve A for an A band and a curve B for a B band will be plotted, as shown in FIG. 2, in which the voltage value V is indicated on the ordinate and the receiving frequency F varied with the voltage is indicated on the abscissa. In the figure, A and B represent, for example, AM and FM bands respectively and W is a band width for each of the bands. While the frequencies for each of the band are different, they are shown at the same positions on the abscissa based on different units. As can be seen from the figure, the varying range for the control voltage required for covering the entire area of the frequency band is a width Va between Va.sub.1 and Va.sub.2 for the A band, and that for the B band is a width Vb between Vb1 and Vb2 Although the coincidence between the curves A and B is possible experimentally, it is difficult to attain such coincidence in case of actual mass production due to scatterings in individual elements and influences by changes in temperature or the like. Then, when the A band is switched to the B band during reception at the point C in the A band, since the control voltage is not changed, the receiving frequency transists to point D thus to exceed the frequency range for the band B. As a preset counter or the like is used for the channel selection, which is adapted to take a minimum value: 00 . . . 0 at the lower limit and the maximum value at the upper limit in each of the bands, the counted value in the counter is switched to the side of the maximum value when shifted out of the range, whereby the frequency further lowers, that is, further apart from the band in the case of the digital channel selection. It is, therefore, necessary to provide a wider idle range for the lower limit (also for the upper limit), or to take such a procedure as manually upwarding the receiving position in the band A so that the switched position may situate above the lower limit E and, thereafter, switching to the digital channel selection.
In the prior art the electronic tuning radio receiver has various types of channel selection systems. While these systems may be provided individually independent from each other, since the variable reactance elements and the control voltage generation section therefor can be used in common, it is customary to employ them in common and provide a multiplexer for utilizing selectively one of them by switching.
In the digital channel selection system, for example, although a high varying speed is desired for the control voltage since it determines a time required for the channel selection, an excessively high speed may cause hunting. Then it is desired to decrease the varying speed in the control voltage as the receiving frequency approaches a set (desired) frequency. In the search channel selection while a high varying speed of the control voltage is also desired since it determines a time required for sweeping the broadcasting frequency band, an excessively high speed provides a problem in the channel selection for a broadcasting station directly adjacent to the present dial position that the adjacent station is passed over in a moment from push to release of the search channel selection button and can not be received. Thus, it is desired to decrease the varying speed in the control voltage just after the depression of the pushbutton. Similar problem is also resulted in the case of the manual channel selection.
Duty ratio in the upward or the downward signal pulse Iu or Id issued from the voltage memory consisting of the integrating circuit is changed in order to change the varying speed in the control voltage, that is, the sweeping speed, and the circuit already proposed comprises a duty changing gate circuit in each of the channel selection systems, which makes the circuit somewhat complicated.
When the channel selection in each of the systems has been completed, the receiver becomes receiving state, wherein AFC (automatic frequency control) is applied to enable the reception always at the best condition. While the AFC is applied by preparing the foregoing positive or negative input voltage based on the output from a discriminator of an S characteristic, hunting may possibly be caused if the duty is not controlled properly.